1. Field of the Invention
The present invention relates to picture image forming equipment using an electrostatic image recording process, such as copying machines, facsimile machines, and printers. The present invention is an improvement on such picture image forming equipment designed to form images on both sides of sheets of paper.
2. Description of the Prior Art
FIG. 18 presents a conceptual drawing illustrating a conventional picture image former 1 applied to a printer. The picture image former 1 performs duplex printing, in which picture images are formed on both sides of the paper. An exposing unit 4 is housed inside the main unit 2 of the picture image former 1. The exposing unit 4 converts image information into an on-off signal for a laser beam and irradiates the laser beam onto a photosensitive unit 3, form electrostatic latent images on the surface of the photosensitive unit 3. A developing unit is arranged on one side of the photosensitive unit 3 and is not illustrated in the FIG. 18. A fixing unit 5 fixes the developed images by applying heat.
A paper transporting channel, in the main unit of the picture image former 1, guides and transports the paper, and is indicated by the dotted line in FIG. 18. The paper transporting channel is composed of an image forming channel A, a paper discharging channel B, and a paper reversing channel C. The three channels A, B, and C merge at a point D to form a connection. The image forming channel A and the paper reversing channel C also merge to form a second connection. The channels are part of separate component parts of the picture image former 1 to be described later. The individual channels A, B, and C are provided with a number of paper transporting rollers set along the channels.
The paper transporting rollers will now be described. A feeding roller 8, having a semicircular sectional shape, draws out one sheet of paper 6 at a time from a paper cassette 7 which contains a number of sheets of paper 6, and transports the sheet of paper 6 into the image forming channel A. An intermediate roller 10 transports the sheet of paper 6 to a registering roller 9. Another intermediate roller 11 transports the sheet paper 6 discharged from the fixing unit 5, and a transporting roller 12 transports the sheet paper 6 discharged from the intermediate roller 11 to the paper discharging channel B. A discharging roller 14 discharges the paper from the paper discharging channel 13 to a paper discharge tray 13. Finally, a transporting roller 15, rotating in the opposite direction from the paper discharging channel B, transports the paper into the paper reversing channel C to the feeding roller 8.
The image former 1 contains an exfoliating corotron 16.
A spring 17 in the paper cassette 7 constantly applies an upward thrust to a plate 18 under the paper 6 to load the paper in the uppermost part of the paper cassette 7 in constant pressurized contact with a separating claw 19.
The operation of the picture image former 1 will now be described.
A duplex image is formed on a sheet of paper a having an upper face b, a lower face e, a fore (heading) edge c, and a rear (trailing) edge d.
As shown in FIG. 19, in which the same parts are represented by the same references marks as those in FIG. 18, when the feeding roller 8 rotates, the sheet of paper a, indicated by a solid line, is moved out of the paper cassette 7. The sheet of paper a is transferred to the image forming channel A, and then the intermediate roller 10 transports the sheet of paper a to the registering roller 9. As illustrated in FIG. 20, images are formed and fixed on the upper face b of the paper a by the photosensitive unit 3 and the fixing unit 5 while the paper a passes through the image forming channel A. Subsequently, the intermediate roller 11 and the transporting roller 12 transport the paper a with an image formed on its upper face b to the paper discharging channel B.
The paper a is then moved into the paper discharging channel B. The paper discharging roller 14, arranged at the downstream end of the paper discharging channel B, discharges the paper a into the inside area of the paper discharge tray 13, as shown in FIG. 21. The paper a is then transported back into the paper discharging channel B with the rear edge d of the paper a set in the forward position, as shown in FIG. 22. The paper discharging roller 14 rotates in reverse to transport the paper a into the reversing channel C, as shown in FIG. 23. As shown in FIG. 24, the transporting roller 15 transports the paper a over the cover 7a of the paper cassette 7 to a position where it is taken up by the feeding roller 8.
The feeding roller 8 is then driven a second time to transport the paper a with an image formed on its upper face b into the image forming channel A for a second time, as shown in FIG. 25. The paper a is then transported by the intermediate roller 10 to the registering roller 9. Images are formed and fixed on the lower face e of the paper a by the photosensitive unit 3 and the fixing unit 5, as shown in FIG. 26. The intermediate roller Il and the transporting roller 12 convey the paper a with images formed on the lower face e to the paper discharging channel B.
The paper a is then transported toward the side of the paper discharge tray 13 by the discharging roller 14 arranged at the downstream end of the paper discharging channel B, as shown in FIG. 27. Finally, the paper a is discharged to the paper discharge tray 13 as shown in FIG. 28.
By this series of steps, images are formed on both faces b and e of the paper a, and the sheets of paper with duplex images are stacked up in the paper discharge tray 13.
The picture image former 1 described above is provided with a gate which opens and closes the image forming channel A. The gate is positioned at the point D, where the three paper transporting channels, the image forming channel A, the paper discharging channel B, and the paper reversing channel C, merge to form a junction. The gate guides the paper a moving in the reverse direction through the paper discharging channel B, as shown in FIG. 22, into the paper reversing channel C.
An actuator for a paper detecting sensor is also provided at the point D to control the timing of the actuation of the paper discharging roller 14 through detection of the paper a as it approaches the point D.
Therefore, the picture image former 1 has a large number of component parts, including a gate and an actuator for a sensor comprised of independent component parts and separately installed at the point D.
In the picture image former 1 described above, accurate formation of an image on the lower face e of the paper a requires passing the paper a through the paper reversing channel C without skew. The conventional picture image former 1 cannot accurately form images on the lower face e of the paper a. Paper skew occurs when the fore edge c runs against the separating claw 19 of the paper cassette 7 while the paper a moving in reverse is guided over the cover 7a of the paper cassette 7.
In the duplex picture image former 1, the discharging roller 14 must rotate in reverse to form images on both faces of the paper. When many sheets of paper are stacked up in the paper discharge tray 13, the equipment is susceptible to paper jams resulting from capture of the paper by the discharging roller 14 out of the paper discharging tray 13. Jamming is more likely when sheets of paper in the paper discharge tray 13 stick together because of static electricity.